diff --git a/js/src/ion/AsmJSLink.cpp b/js/src/ion/AsmJSLink.cpp
index 8bc2c3794c06..46e87e0841b1 100644
--- a/js/src/ion/AsmJSLink.cpp
+++ b/js/src/ion/AsmJSLink.cpp
@@ -31,7 +31,7 @@ LinkFail(JSContext *cx, const char *str)
 }
 
 static bool
-ValidateGlobalVariable(JSContext *cx, const AsmJSModule &module, AsmJSModule::Global global,
+ValidateGlobalVariable(JSContext *cx, const AsmJSModule &module, AsmJSModule::Global &global,
                        HandleValue importVal)
 {
     JS_ASSERT(global.which() == AsmJSModule::Global::Variable);
@@ -71,7 +71,7 @@ ValidateGlobalVariable(JSContext *cx, const AsmJSModule &module, AsmJSModule::Gl
 }
 
 static bool
-ValidateFFI(JSContext *cx, AsmJSModule::Global global, HandleValue importVal,
+ValidateFFI(JSContext *cx, AsmJSModule::Global &global, HandleValue importVal,
             AutoObjectVector *ffis)
 {
     RootedPropertyName field(cx, global.ffiField());
@@ -87,7 +87,7 @@ ValidateFFI(JSContext *cx, AsmJSModule::Global global, HandleValue importVal,
 }
 
 static bool
-ValidateArrayView(JSContext *cx, AsmJSModule::Global global, HandleValue globalVal,
+ValidateArrayView(JSContext *cx, AsmJSModule::Global &global, HandleValue globalVal,
                   HandleValue bufferVal)
 {
     RootedPropertyName field(cx, global.viewName());
@@ -102,7 +102,7 @@ ValidateArrayView(JSContext *cx, AsmJSModule::Global global, HandleValue globalV
 }
 
 static bool
-ValidateMathBuiltin(JSContext *cx, AsmJSModule::Global global, HandleValue globalVal)
+ValidateMathBuiltin(JSContext *cx, AsmJSModule::Global &global, HandleValue globalVal)
 {
     RootedValue v(cx);
     if (!GetProperty(cx, globalVal, cx->names().Math, &v))
@@ -137,7 +137,7 @@ ValidateMathBuiltin(JSContext *cx, AsmJSModule::Global global, HandleValue globa
 }
 
 static bool
-ValidateGlobalConstant(JSContext *cx, AsmJSModule::Global global, HandleValue globalVal)
+ValidateGlobalConstant(JSContext *cx, AsmJSModule::Global &global, HandleValue globalVal)
 {
     RootedPropertyName field(cx, global.constantName());
     RootedValue v(cx);
@@ -216,7 +216,7 @@ DynamicallyLinkModule(JSContext *cx, CallArgs args, AsmJSModule &module)
         return false;
 
     for (unsigned i = 0; i < module.numGlobals(); i++) {
-        AsmJSModule::Global global = module.global(i);
+        AsmJSModule::Global &global = module.global(i);
         switch (global.which()) {
           case AsmJSModule::Global::Variable:
             if (!ValidateGlobalVariable(cx, module, global, importVal))
diff --git a/js/src/ion/AsmJSModule.h b/js/src/ion/AsmJSModule.h
index 33d4be7af899..ae5d1e89a5de 100644
--- a/js/src/ion/AsmJSModule.h
+++ b/js/src/ion/AsmJSModule.h
@@ -62,7 +62,7 @@ class AsmJSModule
                 uint32_t index_;
                 VarInitKind initKind_;
                 union {
-                    Value constant_;
+                    Value constant_; // will only contain int32/double
                     AsmJSCoercion coercion_;
                 } init;
             } var;
@@ -79,6 +79,8 @@ class AsmJSModule
         void trace(JSTracer *trc) {
             if (name_)
                 MarkString(trc, &name_, "asm.js global name");
+            JS_ASSERT_IF(which_ == Variable && u.var.initKind_ == InitConstant,
+                         !u.var.init.constant_.isMarkable());
         }
 
       public:
@@ -370,6 +372,7 @@ class AsmJSModule
     }
 
     bool addGlobalVarInitConstant(const Value &v, uint32_t *globalIndex) {
+        JS_ASSERT(!v.isMarkable());
         if (numGlobalVars_ == UINT32_MAX)
             return false;
         Global g(Global::Variable);
@@ -451,7 +454,7 @@ class AsmJSModule
     unsigned numGlobals() const {
         return globals_.length();
     }
-    Global global(unsigned i) const {
+    Global &global(unsigned i) {
         return globals_[i];
     }
     unsigned numFuncPtrTableElems() const {